1. Field of the Invention
The invention relates to a pixel circuit for a display, and more particularly to a dual-pixel circuit for an active-matrix organic light emitting display (AMOLED).
2. Description of the Related Art
FIG. 1 illustrates a conventional pixel circuit for an organic electroluminescent display (not shown). The conventional pixel circuit is formed in a region where a scan line 101 intersects a data line 103, and includes a driving circuit 10, and an organic light emitting diode (OLED) 11.
The OLED 11 has an anode, and a cathode receiving a low-level bias voltage (Vs).
The driving circuit 10 includes first and second thin film transistors (TFT) 12, 14, and a capacitor 13. The first TFT 12 has a first end for receiving a high-level bias voltage (Vd), a second end coupled to the anode of the OLED 11, and a control end receiving a control voltage (Vg12) such that the OLED 12 is operable between an ON-state and an OFF-state in response to the control voltage (Vg12). The capacitor 13 is coupled between the first end and the control end of the first TFT 12. The second TFT 14 has a first end connected to the data line 103 to receive a data voltage (Vdata) supplied therefrom, a second end coupled to the control end of the first TFT 12, and a control end connected to the scan line 101 to receive a scan voltage (Vscan) supplied therefrom such that the second TFT 14 is operable between an ON-state and an OFF-state in response to the scan voltage (Vscan).
When the second TFT 14 is operated in the ON-state, the data voltage (Vdata) is transmitted to the control end of the first TFT 12 through the second TFT 14, and serves as the control voltage (Vg12) such that the capacitor 13 is charged to thereby obtain a voltage across the capacitor 13 equal to Vd−Vdata. Thereafter, when the second TFT 14 is switched from the ON-state to the OFF-state, the voltage across the capacitor 13 is maintained. Thus, if a potential difference between the first end and the control end of the first TFT 12, i.e., the voltage across the capacitor 13, is greater than a threshold voltage of the first TFT 12, the first TFT 12 produces a driving current (Io) that is proportional to the voltage across the capacitor 13 and that flows to the OLED 11 to control brightness gray levels thereof. Since the OLED 11 is continuously provided with the driving current (Io), the OLED 11 easily degrades to thereby reduce the lifetime thereof. As such, the AMOLED cannot substitute for an LCD display.
Referring to FIG. 2, a hysteresis effect on the first TFT 12 is shown to illustrate the relationships between the control voltage (Vg12) and the driving current (Io) in accordance with a hysteresis loop (L). For example, it is assumed that the first TFT 12 can be driven to conduct when the control voltage (Vg12) is at Vdrive. However, due to the hysteresis effect, the driving current (Io) becomes I1 when the control voltage (Vg12) transfers from a high potential corresponding to a point (P1) on the hysteresis loop (L) to a relatively lower potential, i.e., Vdrive, corresponding to a point (P2) on the hysteresis loop (L) or becomes I2 when the control voltage (Vg12) transfers from a low potential corresponding to a point (P3) on the hysteresis loop (L) to a relatively higher potential, i.e., Vdrive, corresponding to a point (P4) on the hysteresis loop (L), wherein I2 is greater than I1. As a result, when the control voltage (Vg12) is at Vdrive, the OLED 11 emits light at various brightness, thereby resulting in non-uniformity of the organic electroluminescent display.
Therefore, improvements may be made to the above techniques.